1. Field of the Invention
The present invention relates to a lithium secondary battery, and more specifically to improvement of a carbon material for a negative electrode of a lithium secondary battery, or to improvement of both a negative electrode and an electrolyte solution of a lithium secondary battery.
2. Description of the Prior Art
In recent years, carbon materials have been studied, instead of conventional lithium alloys, for the use of negative electrode material for lithium secondary batteries because 1 they have high flexibility and 2 they do not cause mossy-shaped lithium to precipitate by electrolysis.
The carbon material that has principally been studied for this purpose is coke (see U.S. Pat. No. 4,725,422), and graphite has hardly been studied. For instance, U.S. Pat. No. 4,725,422 discloses a secondary battery comprising for the negative electrode a carbon material having the spacing of (002) planes, d.sub.002, of at least 3.37 .ANG. and the crystallite size in the direction of c axis, Lc of not more than 220 .ANG.. The above carbon material is a kind of coke.
Coke, however, hardly provides large-capacity batteries, since the amount of lithium introduced with coke negative electrode is not sufficiently large.
To the best of the knowledge of the present inventors, the literatures that propose a secondary battery having a negative electrode comprising graphite are only U.S. Pat. No. 4,423,125 and U.S. Pat. No. 5,130,211.
The above U.S. Pat. No. 4,423,125 discloses a secondary battery comprising for the negative electrode a carbon material having occluded lithium as an active material and as an electrolyte solution a solution of an electrolyte solute of LiAsF.sub.6 dissolved in a solvent of 1,3-dioxolane. According to the USP, a secondary battery having excellent cycle characteristics can then be obtained.
The above known secondary battery is, however, inferior in many features such as capacity per unit weight of graphite (mAh/g), initial charge and discharge efficiency (%), battery capacity (mAh), self-discharge rate (%/month) and charge and discharge efficiency (%), not to mention cycle characteristics (cycle life), as shown by the data for the "conventional battery" in the later-described Examples. The battery therefore is not sufficiently satisfactory for practical purposes.
This is considered to be due to polymerization of 1,3-dioxolane in the negative electrode side (reduction side).
The above U.S. Pat. No. 5,130,211 discloses a secondary battery comprising for the negative electrode a carbon material having a degree of graphitization greater than about 0.40 .ANG., i.e. the spacing of (002) planes, d.sub.002 smaller than about 3.412 .ANG.. The above known secondary battery , however, is not necessarily excellent in the features mentioned above.